So essentially, with Ireland being within the Common Travel Area, one does not need to pass through UK immigration upon landing in the UK, but still do need to pass through UK customs. I suspect the gate you arrived at did not have the appropriate routes/access to lead you past immigration (but not before customs). Slightly digressing but I've always found it curious as to why at DUB, UK arrivals are subject to Irish passport control but not in reverse! If anyone else here knows the reason, I'd be curious to know
Avios is no longer being given as compensation from multiple reports here.